In present logic and dynamic random access memory devices, there are several levels of metalization so as to propagate the desired signals from one portion of the device to another. With the increased levels of metalization, the decrease in size of these devices, and the need for higher performance, several problems have arisen. The resistance and capacitance of the levels of metalization and their interconnects lie at the heart of most of these problems. Hence, there is a need to reduce the resistance of these structures and the capacitance between these structures.
In addition, cost, yield, and thermal budget are becoming increasingly important constraints for metalization technology. For via and contact level metallization, aluminum plugs have recently received considerable attention, because of their superior performance and low cost relative to tungsten plugs, which were commonly used in prior generations of devices. These performance benefits include: lower resistance, improved electromigration reliability, and extendibility to smaller feature sizes. A substantial reduction in cost of ownership for the back-end-of-line process is enabled by aluminum plug technology via the ability to deposit the plug and interconnect simultaneously, without the numerous processing steps required for tungsten plugs. By avoiding the need to etchback tungsten or perform chemical-mechanical polishing (CMP) on tungsten, aluminum plug processing eliminates several costly equipment steps and can dramatically reduce yield loss by reducing the number of processing steps.
Aluminum plugs have been formed using high temperature reflow processing. See Z. Xu et. al, PROCEEDINGS OF VMIC, 158 (1994). In addition, aluminum plugs have been formed using high pressure reflow processing. See G. A. Dixit et. al., IEDM TECHNICAL DIGEST, 105 (1994). High temperature reflow includes deposition of a wetting layer and a two step cold/hot physical vapor deposition (PVD) aluminum deposition. This technique requires high temperatures, is sensitive to outgassing and surface conditions, and is prone to barrier failure at these excessive temperatures. High temperature reflow also fails for vias smaller than 0.35 microns without sloped sidewalls, and the temperatures required for this process are greater than most low dielectric constant materials are able to withstand.
By extruding aluminum into the contact/via at high pressure and a wafer temperature of around 400 C, high pressure aluminum plug fill has overcome most of the problems associated with hot reflow. Unlike high temperature reflow, high pressure reflow works better with steep sidewalls and small dimensions, provided a continuous TiN wetting layer is deposited in the via. See W. Y. Hsu et. al., ADVANCED METALIZATION FOR ULSI, 265 (1995). However, this method of fabrication suffers from the following disadvantages: complicated processing equipment for the high pressure regime, questionable reliability of high-pressure equipment, and sensitivity to wafer outgassing.
CVD of aluminum films has also been demonstrated for plug-fill, but several problems have previously limits its use. These problems include: rough morphology, lack of strong &lt;111&gt; texture, no means of copper doping of the aluminum film, risk of voids in the aluminum film, and high precursor cost Roughness, as measured by decreased reflectivity, of a film increases dramatically with increasing thickness. This roughening effect can be reduced by the choice of different nucleation layers, but aluminum films often grow on these layers with randomly oriented texture or form intermetallic phases that are deleterious to interconnect performance (such as TiAl.sub.3) during subsequent thermal processing. The most serious problem using blanket CVD aluminum films for plug fill is the risk of void formation during the deposition process. Seams formed during CVD aluminum plug-fill will form a spherical void during later processing and will not, generally, anneal out because of trapped gas from the CVD process. The presence of such defects results in a high risk for open circuits and electromigration failure. Copper incorporation in the CVD Al film, needed to improve electromigration lifetime, has not been practical using codeposition due to the chemical incompatibility of commercially available Al and Cu precursors. Fortunately, PVD deposition of Al--Cu onto CVD Al has been demonstrated to be an effective method for doping CVD Al with Cu, because the rapid diffusion of Cu in Al at modestly elevated temperatures (i.e. greater than 300 C) provides for uniform distribution of Cu.